In tape or film drive systems, such as magnetic tape and film recorders or playback devices, a web is often drawn from a supply roll onto a take-up roll, and vice versa, all within a single container. Frequently, in cartridge systems, a transducing head interfaces with the web through an opening in the enclosure. For purposes of this application, magnetic or other tape rolls and reels, film on reels, printing ribbons on spools and other flat, rolled webs of material are all referred to as "tape".
In tape cartridges, particularly magnetic tape cartridges, it is important that the tape web not be allowed to run off the supply or take-up rolls in the forward or reverse directions respectively. In low cost magnetic tape cartridges a strong leader and trailer is attached to the beginning of tape (BOT) and end of tape (EOT), with the other end of the leader and trailer attached mechanically to the supply and takeup rolls respectively. Upon reaching EOT or BOT the drive mechanism is stalled. In computer systems, the high torque required for rapid acceleration makes it impractical to stall the drive at EOT and BOT. Further, a load-point (LP) indication and early warning (EW) indication is required within the body of the recording web before the end of the web is reached.
There have been a number of methods employed in the prior art for the introduction and the optical reading of indicia at EOT, BOT, LP and EW control positions on a magnetic tape. The precise position of such indicia has been specified for certain size cartridges by the American National Standards Institute, for example in ANSI X3Bl/625. As described in U.S. Pat. No. 3,861,619 to Wolff, methods of making tape having control position indicia include bonding or splicing strips, conductive strips, or transparent leaders to the tape. However, Wolff teaches that these methods suffer from a number of defects. First, thickness is added to the tape in the area of attachment and the added thickness may produce unreliable recording. Second, the area of attachment to the tape collects and carries debris to a read/write head and, therefore, can cause dropouts. Third, the area of attachment is subject to tension and may wear out. Finally, Wolff teaches that the reflective strips, conductive strips, and transparent leaders do not provide unambiguous indication of tape position.
Consequently, Wolff teaches punching holes through the tape at control positions to indicate EOT, BOT, EW and LP. The Wolff method does not require areas of attachment between magnetic tape and strips or leaders. However, the punched holes of Wolff do not eliminate the risk of debris contaminating the tape or the read/write head of a tape drive. Nor is unreliable recording entirely eliminated in subsequent layers near to the hole, especially at high bit densities on thin tape wound under high compression wherein embossment is caused in subsequent layers, causing head-to-tape separation and loss of data. Moreover, the holes weaken the tape. Consequently the size of the holes must be relatively small. The size of transparent markings at the control positions of a magnetic tape is an important factor in the manufacture of the tape. The hole size must be sufficiently large to distinguish the transparent markings "signal" from light "noise" projected through pinholes in the tape. Tape pinholes are caused by coating imperfections in minute areas. These imperfections may be a partial or complete reduction in coating thickness which will be detected as noise by sensors employed for sensing of the punched holes.
The "ideal" drive-sensing system collects light only over the solid angle projected through the indicia holes, in which event the optimized signal-to-noise ratio (SNR) may be as low as the ratio of hole transmissivity to background transmissivity. ANSI specifies a maximum background transmissivity of 5%, assuring a theoretical minimum signal-to-noise ratio of 20:1 for the optimized drive. ANSI does not standardize the drives, however, and most fall far short of the ideal, and do not attain this ratio for two reasons. In drives without noisemasking apertures over the sensors, a solid angle of up to three times that from the marker hole has been routinely observed. This increases the detected noise energy by an area factor of up to nine, reducing the theoretical signal-to-noise ratio from 20:1 to as low as 2.2:1. Further, virtually all drives employ a single light source. The marks will not be on the axis of the beam, while a worst-case noise pinhole will often occur on-axis. The signal-energy-to-worst-case-noise-energy ratio will be further degraded by the off-to-on-axis-beam-intensity ratio. For a typical ratio of 0.8, the overall signal-to-noise ratio is attenuated to 1.76:1. Thus, if only minimal ANSI standards are observed, many drives would have inadequate margins of safety.
Consequently, background transmissivities of considerably less than 5% have been found necessary by cartridge manufacturers to provide reliability using the punched hole method of marking and sensing. A decrease in background transmissivity has been accomplished by back-coating the tape. This adds thickness, is a source of contamination, and adds cost to the tape. In cartridges such as described in U.S. Pat. No. 4,172,569, assigned to the assignee of the present invention, wherein tape tension is essentially independent of drag friction, back-coating of tape serves little use other than to attenuate light noise. An object of the present invention is to provide an adequate safety margin for detecting tape markers without requiring tape transmissivity substantially less than that standardized by ANSI.
One method of making large transparent markings on tape, without thickening or weakening the tape, is to utilize solvent or chemicals to remove the opaque coatings on the opposed sides of the transparent tape substrate. It has been discovered that while this method has advantages over the above-described prior art methods, the solvent is not suitable for low-cost, highvolume production.
Another object of the present invention, then, is to increase the signal level by increasing the size of the transparent markings on the tape while avoiding the disadvantages of leaders and holes. The optical elements utilized in the reading of control position indicia are also important to maximization of the signal-to-noise ratio. In drives with optimized sensing, increasing the area of the indices alone will not increase signal level, since the noise-masking aperture over the sensors will not subtend the increased solid angle, nor will the light noise be attenuated. Only drives without a noise-masking aperture would benefit. In the preferred embodiment of the present invention, optics are provided in the cartridge to ensure that the light from the enlarged area will be converged through any drive mask aperture, so all drives will benefit.
In U.S. Pat. No. 4,380,032, assigned to the assignee of the present invention, Pfost discloses an optical element having a structure such that different portions of the width of the tape can be scanned, yet light from each of these different tape portions is at least partially isolated in different tape paths with respect to other portions. The light paths in the optical element direct light from these different tape portions out of the container to a number of light sensors corresponding to the number of light paths. The sensors are connected to a feedback circuit, including a low pass filter, for discriminating between slow contrast changes characteristic of background phenomena and fast contrast changes characteristic of the color markers taught by Pfost. Maximum signal light is thus collected and sensed in the optimum region of sensitivity on the detector curve (between points a and c on FIG. 9 hereof), while light noise is suppressed to a low point of sensitivity (below point c on FIG. 9). However, although the Pfost method of marking and sensing provides for compatibility with all existing drives, only drives with the special circuitry will enjoy the improved signal-to-noise ratio. A further object of the present invention is to suppress noise to a low point of sensitivity without requiring any change in existing drives.
Optical elements may be particularly important in reading cartridges of the various tape length standards. 600 foot (183 meter) cartridges standardized by ANSI have early warning and load point markers which are circular and which have a diameter of 0.046 inches (1.17 mm). The diameter of these markers allows tape drives to distinguish the cartridges from the original 300 foot (91.5 meter) standard cartridges with markers 0.023 inch (0.58 mm) in diameter by the pulse-width of the sensor output. The present invention preserves this standard, while in one preferred embodiment shaping the noise solid angle to an oblong so that drives without noise-masking apertures will enjoy the same SNR enhancement as those employing masks.